1. Field of the Invention
The invention generally relates to heat dissipation layers for semiconductor devices, and more particularly to a semiconductor device with a metal layer above and beyond the region confined by the optical mode of a semiconductor laser which effectively transports heat away from that region and into the heat sink.
2. Description of the Related Art
Within this application several publications are referenced by Arabic numerals within brackets. Full citations for these publications may be found at the end of the specification immediately preceding the claims. The disclosures of all these publications in their entireties are hereby expressly incorporated by reference into the present application for the purposes of indicating the background of the invention and illustrating the general state of the art.
During semiconductor laser continuous-wave (cw) operation, significant heat generation occurs within the active region of the semiconductor device. These devices are typically mounted onto temperature-controlled heat sinks to keep them from heating past their maximum operating temperature. Thermal coupling efficiency between the semiconductor laser structure and heat sink plays a significant role in determining the maximum operating temperature of a semiconductor laser (as measured at the heat sink). The technique of mounting the laser epi-side down on a heat sink has conventionally been the most effective method of heat removal in these devices. However, the epi-side down process is more complicated and has a significantly lower yield than the epi-side up process. Therefore, the epi-side up process is the preferred technique for manufacturing these semiconductor devices.
Ridge waveguide semiconductor lasers employ etching techniques to define mesa structures. These mesas improve laser performance by confining injected carriers and optical modes to the same region of space. The mesa is typically etched down through the laser structure's active region or even partially down through the lower cladding region.
Mid-wave (3-5 μm) and long-wave (8-12 μm) infrared (IR) semiconductor diode lasers (SDLs) are needed as sources for several civilian and military applications. These include local and remote chemical sensing, free-space optical communications, IR countermeasures, and IR ladars. For most applications, lasers are required that can operate under cw (or quasi-cw) conditions at ambient temperatures or at temperatures (T) accessible to thermoelectric coolers (T>approximately 240 K).
Despite the huge variety of commercially available SDLs emitting in the near-IR spectral region (λ<approximately 1.6 μm), the options available at longer wavelengths (greater than approximately 2 μm) are few, usually unsatisfactory, and progressively worse as the wavelength increases. The technical reason for this circumstance is related to the reduced photon energy and the wavelength dependence of various loss and recombination mechanisms in SDLs. Until recently, there seemed to be little hope for significant increases in the cw operating temperatures of SDLs emitting at wavelengths greater than approximately 3 μm.
This circumstance changed with the demonstration of the quantum cascade (QC) laser[1] and the type-II interband cascade (IC) laser[2]. Both of these devices use multi-layered III-V semiconductor structures that provide for electronic states between which lasing can occur with emission, in principle, at any wavelength greater than approximately 2.5 μm (IC) and approximately 3.5 μm (QC). Using band-gap engineering, they can be designed in ways that substantially reduce the significance of mechanisms (including Auger recombination) that plague more conventional SDLs. Moreover, the active regions of these lasers are cascaded in series allowing for a more effective use of applied bias. This leads to quantum efficiencies that can exceed the conventional limit of unity. Both of these attributes have led to the demonstration of improved power conversion efficiencies at the longer wavelengths compared with more conventional SDLs, and to date, encouraging results have been obtained using both designs. Examples of both the QC lasers and IC lasers are well known in the semiconductor industry[3].
An important additional advantage of the IC design is its use of active regions that rely on interband optical transitions (rather than the intersubband transitions used in the QC laser). This leads to higher radiative efficiencies and significantly reduced threshold currents. Calculations have predicted that IC lasers should be capable of operating under cw conditions up to room temperature with high output power[4].
Since its first demonstration, IC laser performance has advanced considerably. Experimental work with these devices has led to IC lasers that have demonstrated high differential external quantum efficiency (>600%) and high peak output power (approximately 6 W/facet at 80 K)[3]. In other conventional designs, high cw wall plug efficiency (>17% at 80 K) and operation at 300 K under pulsed conditions have been obtained[5].
Despite the achievement of lasing at 300 K, significant issues limiting high-temperature performance remain. Principal among these are the rapid increase of non-radiative processes with increasing active-region temperature (e.g., Auger processes) and the large accumulation of heat in the laser's active region at increased operating temperatures. The latter effect leads to a significant temperature offset, ΔT, between the heat sink and active region temperatures under cw conditions. For example, in the IC laser that operated at 300 K under pulsed conditions, the maximum cw heat sink temperature was limited to approximately 150 K. This suggests that ΔT can be quite large. The active/injection regions of this device are surrounded by short period AlSb/InAs strained layer superlattice (SLS) regions that form cladding layers required to confine the laser's optical mode. These SLS regions consist of many interfaces (often>700) that impede the flow of heat out of the laser's core region, and they are partly responsible for a large ΔT.
For most diode lasers, including IC lasers, the behavior of the threshold current density Jth with active region temperature Ta follows the empirical relation Jth=J0 exp(Ta/T0), where J0 and T0 are fitting parameters. The heat sink temperature Th is related to Ta by Ta=Th+RIV, where R is the device thermal resistance and V and I are the device's cw voltage and current, respectively. Here it is assumed that all dissipated energy goes into heat. One can combine these results to determine, for a given value of the thermal resistance R, the maximum heat sink temperature at which the device will operate under cw conditions[6].
This leads to Thmax=T0[Ln(T0/RspVthJ0))−1], where Vth is the threshold voltage across the device at the maximum cw operating temperature, and Rsp is the specific thermal resistance of the device defined by the product of R with A, the device's active region area (cavity length×mesa width). Examination of this expression shows that the maximum temperature of cw operation can be increased by raising T0, the laser's characteristic temperature, and/or by lowering Rsp, J0, and/or Vth. These representations are equally valid when applied to the QC laser or any mid-wave or long-wave semiconductor laser where heat removal is essential because of the higher losses that plague the performance of semiconductor lasers at the longer wavelengths.
FIG. 1 shows the typical cross-section of a conventional device 100 consisting of the IC laser's cascaded active/injection region 130 (or core region) sandwiched between SLS optical cladding regions (with typical thicknesses shown in FIG. 4) 120, 140. In the conventional device 100, the mesa (active region 130 and upper and lower cladding regions 140, 120, respectively) is defined by etching down through the core region 130 to the top of the lower cladding 120. The conventional sample 100 is then covered with a thin electrically insulating layer 160 of SiO2 (or some other typical dielectric material) with a window left open on top of the mesa for electrical contact. The device processing is completed with the evaporation of an Au/Ti (or similar) contact layer 170, 180 on the bottom of the substrate 110 (separated by a bonding layer 190) and on top of the conventional sample 100 extending beyond both sides of the mesa 130 above the insulating SiO2 layer 160. This Au/Ti contact layer 170 rarely exceeds approximately 0.5 μm in thickness, and the mesa 130 is typically approximately 15 μm wide but can range in width between approximately 2 μm to several hundred microns.
The conventional sample 100 is then cleaved and soldered (with uncoated facets, epi-side-up) onto a Cu heat sink 195, and wires 198 are bonded to the metal contact 170. As is evident in FIGS. 1 and 4, the active region 130 is separated from the heat sink 195 by approximately 80-90 μm of a GaSb substrate 110 and approximately 2-3 μm of the SLS cladding layer 120. The substrate 110, which includes a buffer layer 115, has a thermal conductivity of approximately 33 W/m K while the lower cladding material's 120 thermal conductivity is approximately 3.6 W/m K[7].
However, the conventional designs do not adequately lower the thermal resistance to optimal levels. Therefore, there is a need for a new and improved semiconductor device to decrease the thermal resistance of IC lasers mounted in a conventional epi-side-up configuration.